This article was originally published in the July issue of COMPRESSORtech2. We only publish a fraction of our magazine content online, so for more great content, get every issue in your inbox/mailbox and access to our digital archives with a free subscription.
By Antonio Pelagotti and Leonardo Baldassarre
LNG production started in the 1960s with the plants in Algeria and Libya, then restarted in the ’80s and boomed in the last 10 years. Since the ’80s, we have seen fast growth of LNG production, starting from 3 metric tonnes per annum (mtpa) for Qatargas Liquefied Gas Co. Ltd. (QG) 1 and Woodside 1, to 7.8 mtpa for large QG projects. After that time, all the new plants have been designed around 5 mtpa. This paper will describe recent developments introduced in the latest LNG projects.
Previous and current train configurations: Recently, aeroderivative gas turbines such as the LM2500 or LM6000 have been used for 5 mtpa main refrigerant compressors, while in the past Frame 7s were used. A typical Frame 7 train’s installed power is around 190 MW just for the main refrigerant compressor. This type of train is installed in Egypt, Nigeria, Malaysia and Indonesia.
The same LNG production can be achieved with an LM2500, even though the process needs more equipment to reach the same installed power and LNG target production. So, to reach the same LNG output, a Frame 7 (Figure 1) needs two process drivers, an LM6000 (Figure 2) needs four and an LM2500 needs six. In the last case, the trains will operate in tandem to keep the production flowing even when one train is under maintenance.
Aeroderivative gas turbines as mechanical drives have been installed in Darwin, Australia, for the first time and now are in operation in Papua New Guinea, Australia and the U.S.
The typical compressor used in a large LNG plant is the beam-type centrifugal. For a specific service (low-pressure mixed refrigerant), a process axial compressor has been used. The main advantage of the axial compressor is its high efficiency and flexibility. Recently, big improvements have been made on adjustable guide vanes. The lifespan of the vanes has improved with maintenance schedules expected to match that of the driver.
The new LMS100 (Figure 3) is a driver that can be used in LNG thanks to its high flexibility and efficiency. The train can consist of the driver plus just two compressors to produce 4 mtpa of LNG.
It is worth noting that the longest refrigeration train in the world is installed in Angola and is configured as a Frame 7 and three centrifugal compressors plus an electric motor (Figure 4). This configuration has a second train (Frame 6 and three centrifugal compressors plus an electric motor) to complete the refrigeration loop. The entire liquefaction train comprises two trains using Frame 7s and two with Frame 6s for a production total of 5.2 mtpa.
The configuration of the Frame 7 with three compressors and an electric motor is very attractive because it can accommodate all the services (mixed refrigerant and propane) on the same shaft for a single liquefaction train. This can then be copied and the liquefaction plant can have a parallel train configuration, granting 365 days of production including maintenance of one driver. The helper motor can be designed to compensate for the gas turbine’s lack of power during the hottest days, granting full production during the year. A helper can also be used as a starter, reducing the gas flared during startup. This adds the disadvantage of complicating plant layout and complexity, but it is easily managed thanks to large engineering, procurement and construction (EPC) and oil company experience.
The most recent trend in LNG plant layout is to have multiple modules of equal capacity, with each module having one driver and one compressor with a single production variable from 0.6 to 1.5 mtpa, depending on driver power.
E-LNG has recently been used for a 5 mtpa plant in the U.S. The motor size is rather large at around 107,000 hp (80 MW). New, high-power gas turbines such as the LM6000PF+ and LM9000 (Figure 5) have been studied and deployed to maximize module capability. In these projects, there is only one process gas, typically a hydrocarbon mixture.
Several projects have been developed for floating liquefied natural gas (FLNG) applications with different liquefaction technologies. In FLNG applications, due to size and weight constraints, single-casing compressors are used, and these are often barrel designs for ease of installation and maintenance. Typical arrangements include a barrel compressor with an upward nozzle and optimized casing design to reduce weight and dimensions. Special tools must be developed to take care of bundle maintenance in offshore situations and must also take into consideration barge motion.
New Compressor Tech
High-pressure-ratio compressor development
Increasing financial pressures are causing a push for a reduction of the capital expenditures in the LNG market. A good way to limit costs is to reduce the volume of equipment installed and/or its size. Some original equipment manufacturers (OEMs) have recently introduced a high-pressure-ratio compressor (HPRC) to fulfill such a need. The main advantage of this equipment is related to the rotor construction, which is not made with a solid shaft piece with impellers assembled by interference fit, but is constructed by combining impellers connected axially with a geared connection (Hirth coupling) plus a tie rod. This design overcomes the speed limits for interference connections and increases the rotational speed of the impeller by approximately 40%. For a train equipped with an HPRC, the gear can be either a standard parallel axis-type design or an epicyclic gear to reach the highest gear ratio.
The impeller for an HPRC can be open- or closed-type, depending on mechanical properties and efficiency.
The HPRC allows approximately a:
- 50% footprint reduction.
- 30% reduction in weight.
- Higher reliability due to auxiliary equipment reduction (because of fewer casings).
- Lower downtime and easier maintenance.
Integrally geared compressors and isothermal compression
Centrifugal compressors used in the oil and gas world can be standard beam (between bearings) or integrally geared (IG) compressors. The main advantage of IG compressors is higher efficiency, because each impeller is working at its best efficiency point, increasing overall efficiency and reducing absorbed power. IG compressors can have coolers installed after each stage, which further improves efficiency.
Another major advantage of IG designs, especially important for LNG, is the higher operating range, as each stage can be fitted with inlet guide vanes (IGVs). In this way, overall efficiency can be maintained for alternate cases such as high ambient temperatures, resulting in higher annual production. This type of compressor can be used for a precooling service where compression can be optimized for each stream going through the machine and also on lower molecular weight services. A similar concept can be applied for standard beam compressors using cooled return channels and diaphragms, even though this complicates the overall arrangement of the compressor.
Liquid-tolerant compressor developments
Following the trend of cost reduction on capital items, new liquid-tolerant compressors (LTCs) have been developed in order to eliminate the use of or reduce the size of the scrubbers. The maximum amount of liquid carryover that standard centrifugal compressors can receive is about 2% of mass flow. Research and development goals are to develop an LTC that can tolerate liquid carryover up to 30% in mass (LMF) or 5% in volume flow (LVF).
This technology can be useful in booster gas compressors from the well to the liquefaction plant, and can reduce scrubbers within the liquefaction plant itself. Floating LNG applications can benefit even more with the elimination of large scrubbers.